Forging method

1. A forging method for forming a heat sink including a base board and a plurality of pin fins integrally formed on at least one surface of the base board by plastically deforming a forging blank by die forging processing, the method comprising: arranging a back pressure applying pin in a fin-forming hole for forming the pin fin in a slidable manner, the fin-forming hole being formed in a forging die, wherein the back pressure applying in includes a concave portion on a restraining tip end face of the back pressure applying pin; applying a back pressure to a metal material of the forging blank flowing into the fin-forming hole with the back pressure applying pin at a time of plastically deforming the forging blank; restraining the metal material with the concave portion on the restraining tip end face of the back pressure applying pin; and integrally forming a joining convex portion on a tip end of the pin fin by the metal material filled in the concave portion.

2. The forging method as recited in claim 1 , wherein the joining convex portion of the pin fin protrudes higher than a thin protruded portion formed at a tip end outer peripheral edge of the pin fin.

3. The forging method as recited in claim 2 , wherein the thin protruded portion is formed by burrs.

4. The forging method as recited in claim 1 , wherein a tip end of the joining convex portion of the pin fin is formed into a flat surface.

5. The forging method as recited in claim 1 , wherein the forging die includes a punch and a die, and wherein the fin-forming hole is provided in the punch.

6. The forging method as recited in claim 1 , wherein the forging die includes a punch and a die, and wherein the fin-forming hole is provided in the die.

7. A production method of a heat sink including a heat transfer plate, the method comprising: a step of obtaining a heat sink by a forging method as recited in claim 1 ; and a step of fixedly joining a heat transfer plate to tip ends of a plurality of pin fins without removing thin protruded portions formed at tip end outer peripheral edges of the plurality of pin fins of the heat sink.

8. A production method of a heat sink including a heat transfer plate, the method comprising: a step of obtaining a heat sink by a forging method as recited in claim 1 ; a correction step of adjusting protruding amounts of joining convex portions of a plurality of pin fins of the heat sink by pressing a corrective punch having a flat pressing surface against tip ends of the plurality of pin fins; and a joining step of fixedly joining a heat transfer plate to the tip ends of the plurality of pin fins after performing the correction step.

9. A correction method of a heat sink, comprising: a step of obtaining a heat sink by a forging method as recited in claim 1 ; and a correction step of adjusting protruding amounts of joining convex portions of a plurality of pin fins of the heat sink by pressing a corrective punch having a flat pressing surface against tip ends of the plurality of pin fins.

10. A forging device for forming a heat sink including a base board and a plurality of pin fins integrally formed on at least one surface of the base board by plastically deforming a forging blank by die forging processing, the forging device comprising: a forging die; a back pressure applying pin arranged in a fin-forming hole of the forging die, wherein a restraining tip end face of the back pressure applying in includes a concave portion that restrains a metal material of the forging blank so that a joining convex portion is integrally formed on a tip end of a pin fin by the metal material filled in the concave portion.